How fast is the solar eclipse? And 32 other questions, answered

On Aug. 21 we will see the first solar eclipse in the U.S. since 1979. Our graphic explains exactly what one is, shows it's path and some how-to viewing tips. By Ramon Padilla, Karl Gelles, Dann Miller, Walbert Castillo, Janet Loehrke and Sara Wise, USA T

Doyle Rice , USA TODAY , TEGNA1:13 PM. CDT July 14, 2017

The sun is seen partially covered by the moon on Easter Island, 3700 km off the Chilean coast in the Pacific Ocean, on July 11, 2010. A total solar eclipse began its 11,000 kilometer (6,800 mile) arc over the Pacific Sunday. (Photo: MARTIN BERNETTI/AFP/Getty Images, 2010 AFP)

The biggest and best solar eclipse in American history is coming soon to a sky near you.

On Aug. 21, a total solar eclipse will be visible from coast to coast. It will be the first total eclipse visible only in the USA since the country's founding in 1776.

Here's everything you need to know about the spectacular event:

What is a solar eclipse?

A solar eclipse occurs when the moon passes between the sun and the Earth. The moon appears to completely cover the disk of the sun. By blocking the sun's light, the moon casts a shadow that turns day into an eerie twilight here on Earth. Eclipses occur because of the special coincidence of the moon and the sun being the same size to our eyes. The sun is 400 times wider than the moon, but it is also 400 times farther away, so they appear to be the same size in the sky.

When is the next solar eclipse?

The next total solar eclipse here in the U.S. will be on April 8, 2024, which will be visible from Texas to New England. More total eclipses in the U.S. will follow in 2044, 2045 and 2078. In other parts of the world, the next total solar eclipse will be visible in Chile and Argentina on July 2, 2019.

When was the most recent total solar eclipse?

In the U.S., it was on Feb. 26, 1979, in Washington, Oregon, Idaho, Montana and North Dakota, a day in which the weather was unfortunately "bleak." The August eclipse will be the first total solar eclipse that will be visible from coast to coast since June 1918. Elsewhere, the most recent total solar eclipse was in Indonesia on March 9, 2016.

Why is this called the "Great American Eclipse?"

This is the first total solar eclipse that's only visible in the U.S. and no other country. It will be the first total eclipse visible only in the USA since the country was founded in 1776.

Why is the total solar eclipse only visible in the U.S.?

Eclipses are only visible in parts of the world at any given time. This one just happens to be in the U.S.

What time is the solar eclipse?

It depends on where you live. The eclipse will start on the West Coast in Oregon and trace a 67-mile wide path east across the country, finally exiting the East Coast in South Carolina. The total eclipse begins in Oregon at 10:16 a.m. PDT. (To be the first person on land to see the eclipse, be on the waterfront at Government Point, Ore., at 10:15:56.5 a.m. PDT.) The total eclipse will end near Charleston, S.C., at 2:48 p.m. EDT.

How long will the solar eclipse last?

At any given location, the total eclipse will last for around 2 or 3 minutes. But from the beginning of the eclipse in Oregon to the end of the eclipse in South Carolina, the eclipse will last about an hour and a half.

Where is the "path of totality?"

The path, in which the moon's shadow sweeps across the Earth's surface, will cross parts of 12 states: Oregon, Idaho, Wyoming, Nebraska, Kansas, Missouri, Illinois, Kentucky, Tennessee, Georgia, North Carolina and South Carolina.

Some of the larger cities directly in the path include Nashville, Tenn.; and Greenville, Columbia and Charleston in South Carolina. Both Kansas City and St. Louis, Mo., are barely outside the path.

The average speed of the moon's shadow as it crosses the U.S. is nearly 1,700 mph. That's over two times faster than a supersonic jet.

How many people will be able see the total eclipse?

An estimated 12 million people live within the path of totality. The number of people within just one day's drive of the totality zone is around 200 million.

What will I see during a total solar eclipse?

During a total solar eclipse, the disk of the moon blocks out the last sliver of light from the sun, and the sun's outer atmosphere, the corona, becomes visible. The corona isn't an indistinct haze; skywatchers report seeing great jets and ribbons of light, twisting and curling out into the sky.

What will happen during the total eclipse?

Plants and animals act as though night is falling, as flowers close up and birds return to roost. The temperature can drop 10 degrees or more.

Where will a partial solar eclipse be visible?

If you're not in the "path of totality," you'll still get a chance to see a partial eclipse, when only a part of the sun is blocked by the moon. A partial eclipse will be visible in all of North America, parts of South America, western Europe and Africa. While a partial eclipse is still cool, you won’t notice your surroundings getting dark.

Could this be the most-viewed eclipse ever?

Astronomy magazine says yes, basing this proclamation on four factors: 1) The attention it will get from the media; 2) The superb coverage of the highway system in our country; 3) The typical weather on that date; and 4) The vast number of people who will have access to it from nearby large cities.

How can you look at the solar eclipse?

The only moment it's safe to look at the eclipse is during the 2-3 minutes when the sun is completely behind the moon. Before and after that — during the partial eclipse — special eclipse glasses, or welder's goggles, must be worn. That's because the sun’s surface is so bright that if you stare at any portion of it, no matter how small, it produces enough light to permanently damage your retina. Our eyes never evolved to look at the sun without suffering severe damage. Regular sunglasses are also not safe to use.

Can you take a photo of the eclipse with your smartphone?

Yes, but the quality may be rather poor, as smartphones were never designed for sun and moon photography. The best thing to do is to cover the camera lens with a solar filter during the moments before (and after) the total eclipse when the sunlight is still blinding. Though it may be OK for a few moments, it's not wise to point your smartphone camera at the brilliant, un-eclipsed sun for an extended period of time without putting a filter over the lens. A telephoto lens system is absolutely a must-have for quality eclipse photography with a smartphone. Most of the best shots you'll see related of the eclipse will be taken with professional digital cameras on tripods, or shot through a telescope.

Will there be "eclipse tourists?"

Yes, the eclipse tourist hype is in full swing: Organizers of the Oregon SolarFest are calling it "a rare, mind-blowing cosmic experience," while Nashville promises visitors "a once-in-a-lifetime celestial event." Meanwhile, Carbondale, Ill., is promoting itself as the “Eclipse Crossroads of America," and Charleston, S.C., is planning a blues-and-barbecue harbor cruise during the eclipse.

Will traffic be bad on Aug. 21?

Yes, it could be one of the worst traffic days in U.S. history, some NASA representatives predict. Although about 12 million people live within the narrow band of totality, about 200 million reside within a day's drive of it. The agency has estimated that the population inside the path of totality may double on the day of the eclipse.

Why is the total solar eclipse only visible in the U.S.?

Eclipses are only visible in parts of the world at any given time. This one just happens to be in the U.S.

What cities have highest likelihood of clear viewing conditions?

Based on historical weather conditions, folks in Idaho, Wyoming and Nebraska will have the best chance of clear skies. Casper, Wyo., has an 88% chance of clear skies on Aug. 21.

What cities have highest likelihood of cloudy viewing conditions?

Both the Pacific and Atlantic coasts could see cloudy conditions. Increased cloud cover will also be possible as the eclipse travels across the country east of the Mississippi River. Both Nashville and Columbia have a 44% chance of clear skies.

Do solar eclipses affect humans?

No, there's no evidence that eclipses have any physical effect on humans. However, throughout history, eclipses have been known to produce profound psychological effects. For millennia, they were sometimes interpreted as portents of doom by virtually every known civilization.

How can "citizen scientists" participate in the eclipse?

NASA has many opportunities for amateur astronomers and lifelong learners to get in on the fun of doing science during the eclipse.

Why are solar eclipses so rare?

It takes three celestial bodies (the sun, moon and Earth), all of which are on various orbital paths, to line up in the exact way at the right time to create an eclipse. On average, a total eclipse is visible from any one spot on Earth about once every 375 years. In the U.S., it takes about 1,000 years for every geographic location in the Lower 48 to be able to view a total solar eclipse. So you must be in just the right place at just the right time to observe a total solar eclipse.

How are eclipses predicted?

Astronomers first must work out the geometry and paths of the Earth and moon as they orbit around the sun, mathematically figuring out the motions of each of the three bodies in three-dimensional space. Astronomers then feed the current positions and speeds of the Earth, moon and sun into a computer, programming it to calculate their future paths in relation to one another and what will be the view from the vantage point of us down here on Earth. Eclipses are specific predictions of where the three bodies will be at an exact time and place. Current eclipse forecasts are accurate to less than a minute in time.

Fewer than 1 in 1,000 people has ever seen a total eclipse of the sun.

How many total solar eclipses are there each year?

There is one somewhere on Earth roughly every 18 months.

When was the first solar eclipse?

Solar eclipses have occurred for billions of years, but no one was here to see or record them. The earliest writings we have showing people paid attention to eclipses in any official way are around 5,000 years ago, based on some inscriptions on a monument in Ireland. Eclipses were also recorded in Ancient China, Babylonia, and Greece. In China, people would light fires or shoot arrows at the sun to try to make it catch fire again. After a total eclipse in 840, Emperor Louis of Bavaria was supposedly so terrified of the event that he died shortly afterwards.

What is an annular solar eclipse?

An annular eclipse occurs when the moon covers the sun's center, leaving the sun's visible outer edge to form a “ring of fire” or "annulus" around the moon.

What is a hybrid solar eclipse?

A hybrid eclipse shifts between a total and annular eclipse: At certain points on the Earth, it's a total eclipse, whereas at other points it appears as annular.

What is a lunar eclipse?

It's when the Earth gets in between the sun and the moon. The Earth's shadow is cast on the moon.